Heretofore, various types of thermostats, or the like, employed a snap disc to, in turn, actuate a related switch. Such a snap disc operates to insure that the switch opens and closes with a snap action rather than with a slow creep-type movement. In some devices of this type the disc is formed of bimetal and snaps between two positions of stability in direct response to predetermined changes in monitored temperature.
Snap discs, particularly in the relatively smaller disc sizes, provide a relatively small snap travel. Consequently, heretofore, relatively high precision is required in order to manufacture the prior art thermostats, employing such snap discs, to insure that the manufacturing tolerances of the various components do not result in a device which fails to operate properly.
For example, many snap disc operated prior art thermostats include a bumper or force transmitting member extending between the switch and snap disc. In the manufacture of such prior art thermostats it is common and accepted practice to partially assemble the thermostatic devices and then gauge the subassemblies to determine the exact length of the bumper or force transmitting member required for that particular subassembly. A bumper is then hand fitted to the required length and the assembly completed. Such a manufacturing procedure is expensive since it requires highly skilled assembly personnel, equipment and in some instances still results in defective assemblies.
The prior art has also proposed a thermostatic device which eliminates the need for such selective fitting of the elements. Such are disclosed, for example, in U.S. Pat. Nos. 3,451,028 and 3,636,622 as being, generally, comprised of a cup-shaped case and a switch carrying body which tightly fits into the case with sufficient friction to temporarily retain the body and case in an assembled position. The body and case are free of surfaces which would limit the exact position of the body with respect to the case. During assembly, as disclosed in said U.S. Pat. Nos. 3,451,028 and 3,636,622, the switch carrying body is moved into the case until it is positioned for proper operation with the particular components of the assembled device. Therefore, variations in the size of each element resulting from manufacturing tolerances are automatically compensated for and it is not necessary to selectively fit the various elements of the device. After the switch carrying body is properly positioned in the case, cement is used to hopefully permanently lock or seal the body to the case.
Such prior art thermostatic devices as represented by the disclosures of said U.S. Pat. Nos. 3,451,028 and 3,636,622 have not been found totally acceptable. For example, it has been found difficult to maintain the required respective sizes of the switch carrying body and of the case so as to result in the required degree of interference fit therebetween. This is further aggravated by the fact that the switch carrying body is usually of electrically non-conductive plastic material, or the like, having a first coefficient of thermal expansion and the case, more often than not, is made of metal having a second coefficient of thermal expansion. When such elements are manufactured and then stored, as in normal inventory, the subsequent day-to-day temperature variations result in the switch carrying bodies and the cases experiencing different rates of thermal expansion or contraction which, in turn, often results in either a loss of the required frictional force, as between the cooperating switch carrying body and case, or such a dramatic increase in the magnitude of the actual frictional force as to render it practically impossible to successfully assemble the respective bodies and cases. Further, it has been found that during actual use of such prior art thermostatic devices, the cement, intended to hold the body and case together, will at times fail and such, in turn, leads to ultimate failure of the thermostatic device.
The invention as herein disclosed and described is primarily directed to the solution of the foregoing and related and attendant problems.